1. Field of the Invention
The present invention relates to an audio circuit.
2. Description of the Related Art
An electronic device having a function of playing back an audio signal, examples of which include CD players, audio amplifiers, car stereo systems, portable radios, portable audio players, and the like, includes a sound processor that provides an audio signal with various kinds of signal processing. FIG. 1 is a block diagram showing a typical configuration of an audio system.
An audio system 100 includes a sound source 102, an analog amplifier 104, an A/D converter 106, a DSP 108, a D/A converter 210, a volume circuit 220, a power amplifier 110, and an electroacoustic conversion element 112.
The sound source 102 is configured as a CD player, silicon audio player, cellular phone terminal, or the like, which outputs an analog audio signal. The analog amplifier 104 amplifies the analog audio signal received from the sound source 102 such that its output matches the input range of the A/D converter 106 configured as a downstream stage. The DSP 108 receives a digital audio signal from the A/D converter 106, and performs predetermined digital signal processing on the digital audio signal thus received. Examples of such signal processing performed by the DSP 108 include equalizing, bass boost, treble boost, monaural/stereophonic conversion, digital volume control, and the like.
The D/A converter 210 converts the digital audio signal, which has been subjected to signal processing by the DSP 108, into an analog audio signal. The volume circuit 220 amplifies the output signal of the D/A converter 210 with a gain that corresponds to a volume value. The power amplifier 110 amplifies the output of the volume circuit 220, so as to drive the electroacoustic conversion element 112 configured as a speaker or headphones.
With such an audio system 100, by inserting the volume circuit 220 as a downstream stage of the D/A converter 210, such an arrangement provides improved noise characteristics when the volume is reduced.
FIGS. 2A and 2B are circuit diagrams each showing a function IC (Integrated Circuit) including the D/A converter 210 and the volume circuit 220. The D/A converter 210 has offset error that is non-zero. Thus, such an arrangement requires a measure for suppressing the occurrence of DC shock noise when the volume is switched. In the configuration shown in FIG. 2A, by inserting a DC block capacitor between the D/A converter 210 and the volume circuit 220, such an arrangement allows the DC noise components to be removed. However, such an arrangement is not capable of removing a noise component having a low frequency that is in the vicinity of the DC level.
In the configuration shown in FIG. 2B, the volume circuit 220 is provided with a soft switch circuit 120. The soft switch circuit 120 gradually changes the volume (attenuation amount) over the period of a given time constant. Such a configuration shown in FIG. 2B requires a long time to remove the DC noise when the volume is switched, which is a problem.